An EFM (Eight-to-Fourteen Modulation) system, where fixed length words are used for storing audio signals on a CD (Compact Disc) is disclosed in the Japanese Patent Disclosure (Tokkai-Sho) 57-48848 (now granted as Tokko-Hei 05-19332). A Run-Length Limited two-to-seven (hereinafter referred to as RLL [2.7]) modulation system adopted for a writable optical disc is disclosed in the Japanese Patent Disclosure (Tokkai-Sho) 50-142131 (or corresponding U.S. Pat. No. 3,689,899).
However, in order to achieve high-density recording in the conventional EFM modulation system and a two-to-seven (hereinafter referred to as [2,7]) modulation system, a short-wave laser had to be developed to fully achieve large capacity storage media for image data etc. Such as laser is currently under development.
In the modulation systems described above, characteristic parameters of the modulated codes after conversion, such as a bit interval T, a minimum inversion interval Tmin, a maximum inversion interval Tmax, a detection window width Tw, a digital sum value DSV, etc., have been established. In particular, for high-density recording, the minimum inversion interval Tmin and the detection window width Tw should be large while the maximum inversion interval Tmax should be as small as possible. The minimum inversion interval Tmin is the most important parameter as it directly affects the shortest bit length that can be reconstructed on an optical disc.
The shortest writable or readable bit length on a disc is determined by an attribute of the light source, i.e., the wavelength of a laser beam generated from an optical head for writing or reading optical data on a disc, as well as the numerical aperture NA of an optical objective lens so that the light spot diameter is as small as possible. Therefore, in order to reduce the bit length and increase the recording density, a laser with short wavelength beam was needed. Also, the numerical aperture NA has to be expanded. However, it was difficult to limit a writable disc bit length to below a fixed value as it was difficult to use them in practice.
Thus, in order to in the present writable and readable bit lengths, developing a modulation system having a larger minimum inversion interval Tmin using the current laser technology is an alternative approach. Further, when the minimum inversion interval Tmin is made large, it sacrifies or deteriorates other characteristic parameters. That is, maximum inversion interval Tmax, detection window width Tw, etc. may be deteriorated. However, the effects can be suppressed through signal processing when data are reconstructed. Therefore, in order to achieve a high density recording in a system utilizing the current optical system, it is indispensable to develop a modulation system having a sufficient length for the minimum inversion interval Tmin.
As described above, conventional digital data modulation systems, such as the EFM modulation system and the [2,7] modulation system, are disadvantageous in that a priority of their performance is placed on simplifying data reconstruction or demodulation. As a result, the minimum inversion interval Tmin is insufficient for realizing a large capacity optical disc intended for image data storage.